[bazel] Add a bazel flag to enable building MLIR with CUDA support (llvm#88856)

This makes it possible to specify
`--@llvm-project//mlir:enable_cuda=true` on the bazel command line and
get a build that includes NVIDIA GPU support in MLIR.

Author: apaszke

utils/bazel/llvm-project-overlay/mlir/BUILD.bazel

@@ -6,6 +6,7 @@
 #   The MLIR "Multi-Level Intermediate Representation" Compiler Infrastructure
 
 load("@bazel_skylib//rules:expand_template.bzl", "expand_template")
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load(
     ":build_defs.bzl",
     "cc_headers_only",
@@ -28,6 +29,18 @@ exports_files([
     "utils/textmate/mlir.json",
 ])
 
+bool_flag(
+    name = "enable_cuda",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "enable_cuda_config",
+    flag_values = {
+        ":enable_cuda": "True",
+    },
+)
+
 expand_template(
     name = "mlir_config_h_gen",
     out = "include/mlir/Config/mlir-config.h",

utils/bazel/llvm-project-overlay/mlir/build_defs.bzl

@@ -6,7 +6,8 @@
 
 def if_cuda_available(if_true, if_false = []):
     return select({
-        # CUDA is not yet supported.
+        # CUDA auto-detection is not yet supported.
+        "//mlir:enable_cuda_config": if_true,
         "//conditions:default": if_false,
     })